1. Field of the Invention
This invention relates to a process for preparing D-mannitol. More particularly, the invention relates to a process for preparing D-mannitol, which comprises isomerization(s) of D-glucose to convert a portion of the D-glucose into D-mannose alone or into D-mannose and D-fructose, followed by catalytic hydrogenation of the resulting mixture of sugars under high hydrogen pressure.
D-mannitol has recently come into wide use as the materials for medicines, foodstuffs and chemical products. Generally, D-mannitol occurs spontaneously in plant bodies, especially in marine plants. D-mannitol is also formed chemically through hydrogenation of D-mannose or D-fructose. The theoretical formation ratio of D-mannitol from D-mannose through the hydrogenation is 100%, while that of D-mannitol from D-fructose is 50% because of half mole of D-fructose used being transformed to D-sorbitol. (In this specification and claims, both "%" and "part" are on the dried matter basis).
2. The Prior Art
D-mannitol is currently prepared chemically in such a manner as 100 part sucrose is inverted (i.e., hydrolyzed) to form a mixture of 52 part D-glucose and 52 part D-fructose and the mixture is hydrogenated to transform it to a mixture of D-sorbitol and D-mannitol followed by crystallization of D-mannitol to separate therefrom. However, according to this process, formation ratio of D-fructose from the sucrose used is 52% at most, so that that of D-mannitol by the hydrogenation reduces, from the above-mentioned theoretical formation ratio (i.e., 50%) of D-mannitol from D-fructose, necessarily to 26% or below, resulting in the yield of crystalline D-mannitol being about 17% based on the initial material, because of a considerable amount of D-mannitol remaining inevitably in the mother liquor at the crystallizing step.
In connection with the above, it is known that D-glucose is isomerized to convert a portion of it into D-fructose and/or D-mannose in the presence of an enzyme or base. The chemical formula of such isomerization of D-glucose is represented as below. (Among these isomerizing reactions of sugars, such ones as the conversion of D-glucose into D-mannose are specifically or restrictedly defined as epimerization.) ##STR1##
Known examples of the isomerization of D-glucose are as follows: When glucose-isomerase is added to an aqueous solution of 100 part D-glucose and the resultant mixture is warmed at a temperature of 65.degree. C for 3 days, there is formed an equilibrium mixture of 54 part D-glucose and 46 part D-fructose. There is further, when a saturated lime solution containing 100 part D-glucose is held at a temperature of 35.degree. C for 5 days, formed an equilibrium mixture of 63.5 part D-glucose, 31.0 part fructose, 2.5 part D-mannose and 3.0 part impurities. As for the latter process, rise in the reaction temperature results in lowering of the formation ratio of D-fructose.
Furthermore, it has been disclosed lately in Angew. Chem., 83(23), 967 (1971) as well as specification of Czechoslovakian patent No. 149,051 that an equilibrium mixture of 75 part D-glucose and 25 part D-mannose is obtained without any formation of D-fructose, when 1 part molybdic acid is added to an aqueous solution of low concentration (i.e., about 17%) of 100 part D-glucose and the resultant mixture is heated at a temperature of 70.degree.-100.degree. C for 5-10 hours with pH 7.0 or below (any adjustment of pH values of the reaction systems is not however conducted in the examples of these literatures, while about pH 5 has been exhibited by the trace-experiments of such examples carried out by us).
However, the equilibrium mixture(s) of D-glucose, D-fructose and/or D-mannose obtained by the isomerizations or epimerization, of D-glucose, as mentioned above, are all, from the content(s) of D-fructose and/or D-mannose thereof, expected to be, when submitted to the preparation of D-mannitol, only equivalent to or below the said mixture of 52 part D-glucose and 52 part D-fructose formed by the current chemical process, in point of the D-mannitol formation ratio therefrom based on the initial material.